Olympus Varimagni finder for sale.
Used with my OM-1 (also works with the Pentax K1000 and other cameras) to allow a right angle view through the view finder and magnify the view as well for better focusing. Highly recommended for framing astrophotos and for more comfort in the field.
I bought it new from B&H back in 1996 I believe (I still have the storage case). Excellent condition. Asking $100.